Electron accelerator control system



Feb. 5, 1946.

- w. F. WESTENDORP' ELECTRON ACCELERATOR com'nog SYSTEM Filed Sept. 10,1943 Fig.3

T Inventor:

3 Willem FT Wstendorp,

3 XML, ,5 y WW 1 His Attorney.

Patented Feb. 5, 1946 ELECTRON ACCEL SYS Willem F. Westendorp signor toGeneral Elec tion of New York ERATOR CONTROL TEM Schenectady, N. Y.,astric Company, a corpora- Application September 10, 1943, Serial No.501,754

Claims.

The present invention relates to apparatus'for accelerating chargedparticles, such as electrons, by means of magnetic induction effects.

Apparatus of the character referred to typically includes a closedvessel and a magnetic system for producing a time-varying magnetic fieldof such space distribution as to confine charged particles projectedwithin the vessel to a circular orbit along which the particles arecontinuously accelerated as the magnetic flux through the orbitincreases in magnitude. When the particles have been accelerated to adesired high velocity. they may be diverted from the accelerating orbitfor producing desired useful effects.

A major problem in the operation of magnetic induction apparatus of thetype p cified consists in the provision of suitable means for divertingorbitally moving electrons after their acceleration has proceeded to thedesired degree. For example, the effective utilization of theaccelerated electrons may be carried out by constraining the electronsto move in a new course in which they will be intercepted by anX-ray-producing target. Heretofore, this desired result has beenattained by contracting or expanding the electron orbit.

In accordance with the present invention, means have been providedwhereby the accelerated beam of electrons may be controllably displacedaxially, for example, upwardly or downwardly from the plane in whichthey are accelerated, in order to bring them intointercepting relationwith a target.

The target then may consist of a flat plate with plane bottom surface ofappreciable extent in the direction of the electrons in order to producesocalled "thick target radiation" which is more efficient than thintarget radiation." The target must be accuratel adjustable parallel tothe plane of the orbit in order for the electrons to penetrate the fullthickness. In the case of orbit contraction or expansion, the targetwould have to be accurately curved in order to obtain the same effect.

In general, such displacement is accomplished in accordance with theinvention by auxiliary field-producing means which near the end of theaccelerating period produces a symmetrical modiflcatn of the magneticfield of such character as to permit axial displacement of the path ,ofgyration of the accelerated particles.

The features which I desire to protect herein are pointed out withparticularity in the appended claims. The invention itself, togetherwith further objects and advantages thereof, may best Cal be understoodby reference to the following description taken in connection with theaccompanying drawing, in which Fig. 1 is a partially sectionalized viewof an induction accelerator embodying my invention; Fig. 2'is a diagramof connections; and Fig. 3 is a conventionalized view of a portion ofthe magnetic structure and the deflecting coils whereby the magneticfield is modified to displace the electron stream.

Referring-particularly to Fig. 1, there is shown in section aring-shaped glass vessel l0 providing within it an annular chamber inwhich electrons may be accelerated to a desired velocity. The vessel I0is sealed and preferably highly evacuated. It may be operated whileconnected to a vacuum pump (not shown). A high resistance coating Ii(indicated by stippling); such as an extremely thin layer of conductivematerial, such as silver, is applied to the interior surface of thevessel to prevent wall charging.

The accelerating mechanism comprises a magnetic structure havingrotationally symmetrical (i. e. circular) pole pieces l2, [3 which arecoaxial with the annular vessel l0. These pole pieces are constituted oflaminated iron held in assembly (by means not shown). The oppositelyfacing central pole faces are of essentially planar character. Neartheir outer edges, the poles are of tapered configuration. Fordecreasing the magnetic reluctance of the path between the opposed polefaces, there is provided an insert in the form of two laminated ironcylinders or disks i4, i5. An externally closed magnetic circuit betweenthe base portions of the pole pieces is provided by iron cores l6, I1,is and I9. The magnetic structure is excited by means of a pair ofseries-connected coils 20 and ii which surround the pole pieces I! andi3 and which produce a time-varying'fiux in the magnetic circuit.

The coils 20 and 2| are shown in Fig. 2 as connected in series with oneanother and across a condenser 22 which is assumed to be of suchcapacity as to resonate with the inductance of the coils at a frequencycorresponding to the desired frequency of operation of the apparatus. Tosupply the losses of the resonant circuit thus formed, the coils 20 and2| may be coupled to primary coils 23, 23' which are directly energizedfrom an alternating-current power source represented by the mains 24. Arelatively small amount of power supplied-to the coils 20 and 2i willserve to maintain the resonant system in excited condition.

Within the closed vessel I il (Fig. 1) and also within the region ofinfluence of the magnetic field produced there is provided a thermionic"cathode 2s, m. 4. This cathode is surrounded tron accelerator of thekind shown in Fig. 1 are well understood and are described in priorpatcuts and publications.

A novel feature of my present invention is shown in Fig. 3. The polepieces l2, l3 here are shown somewhat diagrammatically. They: aresurrounded by reversely wound series-connected coils 30, 3| which whensuitably energized displace axially the annular electron stream in the.evacuated chamber ID. The course of energiz-v ing current in the coils39, 3| and the supply conductors 32 is indicated in Fig. 3 by ,arrows tobe in opposite direction in the series-connected coils.

A system of electrical connections, comprising also a feature of myinvention, whereby the coils 39, 3| are excited when the electrons havebeen accelerated, is shown in Fig. 2. The coils 39, 9| receive animpulse of current from the condenser 33 which is charged through athermionic recti fier tube 34, charging current is received from thesecondary of a transformer 35, the primary winding of which is suppliedfrom the mains 2 4. The discharge circuit 36 of the condenser 33contains an ignitron 38, that is, a mercury cathode trigger tube whichmay be rendered conductive at a predetermined instant. It is'renderedconductive by a discharge received from another condenser 39 through theconductors 40, 4|. The condenser 39 is charged by a transformer 42through a rectifier 43 as indicated. The conductor 4| contains aThyratron tube 45, connected in series with an inductor 31 and aresistor 31'. The grid circuit 46 of the Thyratron 45 is connected tothe secondary winding of a peaking'transforme 41 and contains a biasingbattery 49 and a resistance 49. The primary winding of a peakingtransformer" is energized by current received through the phase shifter50. which comprises as usual a capacitor II and a variable resistance52. When the grid I3 of the Thyratron 45 receivesa positive charge animpulse of current is conducted through the Thyratron 45 from thecapacitor 39. This impulse discharge renders the ignitron 39 conductiveand energizes the coils '30, 3|.

The discharge of current through th reversely connected coils 30, 3| isconducted through a saturating reactor 54 which may have a reactancecoil 55 connected inshunt thereto.

The reactor 59 has a high inductance at low values of discharge currentand a negligible inductance at highvalues. It tends to prolong thedischarge through tube 39 after the current drops below saturation ofthe reactor 34. The reactor II permits the current in the reactor 54 toreverse and therefore nearly doubles the time during which reactor 54 isoperating in its non-saturated range, giving nearly double the time forthe ignitron 39 to become deionized. l

Assuming the magnetic field generated by the coils 39, 3| to be sooriented as to lift the accelerated electron stream, X-rays will begenerated by the impact '01 the electrons on target 91, Fig. l. Theelectron stream may be otherwise utilized in a desired manner.Similarly, the stream oi electrons can be displaced downwardly byappropriate reverse excitation of the coils 39, 3|.

the orbit of said electrons, and means forenergizing said modifyingmeans at a predetermined point of the cycle 01' the time-varying field.

2. An electron accelerator comprising means fo producing a stream ofelectrons moving in a circularorbit, means for impressing on said orbita time-varying magnetic field for accelerating said electrons, electricwindings located above and. below said electron stream, said coils beingconnected in series and wound in opposite directions, means'forenergizing said coils to displace in an axial direction the orbit of aidelectron stream, and means for timing the energization of said coilswith said time-varying field.

3. An electron accelerator comprising an annular evacuated chamber,means for producing an electron discharge therein, means for producingatime-varying magnetic field oriented to accelerate said electrons in adefinite orbit in said chamber, electric coils connected and wound togenerate a magnetic field whereby said orbit may be axially displaced,an electric circuit connected to said coils containing a source ofcurrent, a trigger discharge device in said circuit, and means foractivating said device at a time when said electrons have received 'adesired acceleration by said time-varying field.

4. An electron accelerator comprising an annular evacuated chamber,means for producing target axially displaced from said orbit, electricwindings oriented with respect to said orbitto generate a magnetic fieldwhereby said electron v stream may be deflected perpendicular to theplane of said windings and upon said target, and means for energizingsaid windings in predetermined time relation to said accelerating fieldto axially displace the orbit of said electrons into a position in whichimpingement-thereof on said target will result.

5. An electron accelerator comprising an annular evacuated chamber, asource of electrons in said chamber, magnetic means for acceleratingsaid electrons in an annular path in said chamber, a target displacedfrom the orbit of said electrons, means for displacing the electronorbit in an axial direction in intercepting relation with said target,and means for energizing said displacing means when a. predeterminedacceleration

